Welding power sources typically convert a power input to a necessary or desirable power output tailored for a specific application. Welding power source and welding/plasma power source, as used herein, include power sources employed in welding, plasma cutting and/or induction heating. Welding (and cutting and heating) power sources typically receive an alternating current (ac) input signal and provide a high current output power signal. The input signal is typically a sinusoidal line voltage signal obtained from a utility source.
There are many types of welding/plasma power sources that provide output power suitable for welding, cutting and heating. Suitable prior art welding/plasma power sources include phase control power sources, converters, (for example, a series resonant converter delivers a sinusoidal output) and inverters. Converter, as used herein, includes a power circuit that receives or provides an ac or dc signal, and converts it to the other of an ac or dc signal, or to a different frequency. Inverter, as used herein, includes a power circuit that receives or provides a dc bus signal that is inverted to be an ac signal.
Generally speaking, an inverter-type power source receives the sinusoidal line input signal from the utility source, rectifies the sinusoidal line input signal, converts the rectified signal into a dc bus signal having a controllable voltage magnitude, and inverts the dc bus signal to provide an ac output. The ac output is rectified to provide dc welding or cutting output power. Often, the voltage magnitude of the dc bus voltage is greater than the peak voltage of the rectified input signal in these inverter-type power sources.
Inverter power sources suitable for welding, cutting and heating include boost, buck and boost-buck power sources, all of which are well known in the art. One such welding/plasma power source is described in U.S. Pat. No. 5,601,741, incorporated herein by reference.
Converters and inverters often include an energy storage device to deliver energy to the dc bus. Stored energy is used to maintain the voltage magnitude of the dc bus signal at a desired level during operation of the welding/plasma power source. The energy provided to a power source must be equal (over time) to the energy used (output energy plus losses). Generally, the power in equals the power used, but an electrical transient, such as that caused by a decrease in power in or an increase in power used may, for very short periods of time, result in more power being used than provided. Energy stored by the energy storage device is used to temporarily make up the difference between energy provided and energy used. The energy "shortfall" is generally for a short period of time because power from a utility source provided to the welding/plasma power source is able to quickly (on the order of one cycle, e.g.) change to the needed level. Generally, welding/plasma power sources have been designed using a dc bus capacitor as the energy storage device. The dc bus capacitor generally has as small a capacitance as possible to save on costs, but large enough to provide sufficient energy for the very short transients.
The power source described in U.S. Pat. No. 5,601,741, which receives its input signal from a utility source, has a dc bus capacitor C3/C7 as the energy storage device. This capacitor is capable of storing approximately 1.37(P)(T) joules of energy where P is the available output power of the welding/plasma power source in watts and T is the period of the ac input signal in seconds (for a 60 Hz input signal, T is approximately equal to 16.67 milliseconds). The amount of energy stored by the dc bus capacitor, and therefore available to the dc bus, is proportional to the size (capacitance) of the capacitor. This prior art welding/plasma power source has a capacitance of 400 microfarads (C3 and C7, which are connected in series, individually each have a capacitance of 800 microfarads), or approximately 2.75(P)(T)/(V.sup.2) farads where V is the magnitude of the dc bus voltage in volts.
However, welding/plasma power sources can also receive their ac input signal from a generator source. Unlike utility sources which are capable of quickly increasing the power in response to the demands of a single welding/plasma power source, generator sources are typically limited in the amount of power they are capable of providing to a welding/plasma power source and in how fast they can react to increased demands for more power. Generator sources may also experience, in response to electrical transients and/or sudden increases in power used (e.g during periods of heavy loading), mechanical transients that are longer in duration than electrical transients.
These limitations, which are not present with utility sources, can reduce the amount of output power that is available from the welding/plasma power source when it is run off of a generator source. The degradation in output power can be as much as 75 percent. Therefore, it is desirable to have a welding/plasma power source that, when run off of a generator source, can provide at least 75 percent of the power that it is capable of providing when run off of a utility source.
Utility source, as used herein, includes sources for which loading by the welding/plasma power source is insignificant or negligible during normal operation of the welding/plasma power source. Generator source, as used herein, includes sources for which loading by the welding/plasma power source is not insignificant or negligible during normal operation of the welding/plasma power source.
It is common for a welding/plasma power source, when powered by a generator source, to place a heavy load on the generator source. This is especially true in plasma cutting applications where the plasma cutter draws a continuous high current output during the plasma cutting operation. In these heavy loading situations, the generator may temporarily slow down (e.g mechanical transient) and not be capable of providing adequate input power to the plasma cutter throughout the entire cutting operation. The voltage magnitude on the dc bus can dip or sag as a result. Without sufficient stored energy available to maintain the voltage magnitude of the dc bus at the desired level during these heavy loading periods, the available plasma cutting output power will be inadequate to maintain a sufficient arc during cutting (or welding or heating).
Recent advances in power factor correction circuitry has allowed welding/plasma power sources to operate much more efficiently than in the past. Power factors of 0.99 are now typical. As a result, less input power to the welding/plasma power source is needed to obtain a given output power. This means that smaller generator sources (e.g. having output power ratings approaching the output power rating of the welding/plasma power source) theoretically can be used to run the welding/plasma power source.
However, although the rated output power from these smaller generator sources may theoretically be adequate, the actual output power from these smaller generator sources during heavy loading may not be adequate for certain applications such as plasma cutting. Furthermore, heavily loading these smaller generators can exacerbate the problems related to mechanical transients.
Prior art energy storage devices are capable of maintaining the voltage magnitude of the dc bus signal at an adequate level when the welding/plasma power source is powered by a utility source. These energy storage devices are not capable, however, of maintaining the voltage magnitude of the dc bus voltage at an adequate level when the welding/plasma power source is powered by a generator source which is heavily loaded such as in plasma cutting applications. Therefore, it is desirable to have an energy storage device that is capable of storing sufficient energy to maintain the magnitude of the dc bus voltage at an adequate level during heavy loading of the generator source.